Generally, brake pressure control devices are equipped with a large number of electromagnetically-actuated valves used as inlet valves and exhaust valves. However, in order to reduce the number of these solenoid valves, a variety of brake pressure control devices have been disclosed in which the inlet valves are hydraulically actuated instead of electromagnetically actuated.
For example, in the device disclosed in Publication of Unexamined Japanese Patent Application Heisei 1(1989)-306357, as shown in FIG. 14, a main brake line 62 connects the master cylinder 60 to a wheel cylinder 61, an exhaust line 63 branches from the main brake line 62 with a hydraulically actuated flow control valve 64 provided at the point of the branching, and a conventional electromagnetically-actuated exhaust valve 65 is provided in the exhaust line 63.
The flow control valve 64 includes an inner cylinder 67 housed inside a cavity 66, a spool 69 housed to slide inside the inner cylinder 67 and forming a fixed-diameter orifice 68 therein, and return springs 70 which energize the spool 69 towards the closed end of the cavity 66. A plural number of fluid ducts are bored through the inner cylinder 67 and spool 69 in the radial direction.
The brake pressure control device of a type as described is subject to certain potential problems.
The flow control valve has a large brake fluid path formed from an inlet port 71, fluid ducts 72-76, and a primary outlet port 77.
In order to rise the pressure again during an antilock braking (ABS), the spool 69 drops thereby blocking the large fluid path, wherein the brake fluid is bypassed through the fixed-diameter orifice 68 of small cross section and supplied to the wheel cylinder 61. The cross section diameter of the fixed-diameter orifice 68 is extremely small, of the order between 0.2 mm-0.3 mm, in which case contaminants can readily accumulate and are removable only by physically dismantling and cleaning the device. A buildup of contaminants will block the fluid flow necessary for the brake pressure to rise, and braking force will be inadequate.
During sudden braking when the spool 69 moves towards the return spring chamber 80, the large fluid path becomes constricted or blocked, creating the danger of insufficient braking force. That is, during sudden braking when a strong fluid force acts on the spool 69, it is pushed towards the return spring chamber 80. The brake fluid inside the return spring chamber 80 is thereby compressed and forced around the exterior of the spool 69 and out through the fluid duct 78 which has passage to the wheel cylinder 61. This allows the spool 69 to drop (that is, move farther towards the return spring chamber 80), constricting or blocking the fluid path between the two fluid ducts 75 and 76, resulting in an insufficient braking force.
In the brake pressure control device as described, a plural number of fluid ducts 72, 73, 75, 76, 78, 79 must be bored in the radial direction through the inner cylinder 67 and spool 69. The fabrication of these ducts increases the cost of the device.